A chemical battery refers to a device for transforming chemical energy to electrical energy, which is becoming a necessity for daily life and work. At present, the commercially available batteries mainly include lead acid battery, nickel-cadmium battery, nickel-hydrogen battery, and lithium ion battery. The lead acid battery and the nickel-cadmium battery are restricted for use in various countries in the world due to the disadvantages of low specific energy, containing toxic metals, and serious environmental pollution. The nickel-hydrogen battery is only suitable for using as the power source of small size devices due to low operating voltage and large self-discharge at high temperature. The lithium ion battery has succeed in portable electronic devices, but the problems regarding insufficient power, high price and safety risk prevent it from meeting the demands for large scale systems.
A nickel-zinc secondary battery is comprised of a zinc electrode and a nickel electrode, and combines the advantages of high negative electrode capacity in zinc-silver battery and long service life of positive electrode in nickel-cadmium battery. A nickel-zinc battery has a relatively high specific energy of actually 115 Wh/kg or higher; a specific power of more than 200 W/kg; a wide operating range, that is, a working temperature range of −20° C.˜60° C.; and the advantages of high operating voltage, stable discharge, supporting high current discharge, low cost, and free of pollution. Having these advantages, a nickel-zinc secondary battery is suitable for using as the power source of portable electronic products such as portable computer, digital camera, and the like, and is expected to become the power battery for electric automobiles, replacing conventional power batteries.
A nickel-zinc secondary battery includes a battery case, an electrode assembly, and an electrolyte, in which the electrode assembly and the electrode are accommodated in the battery case. The electrode assembly comprises a nickel positive electrode, a zinc negative electrode, and a membrane separator disposed between the nickel positive electrode and the zinc negative electrode. The currently existed nickel-zinc secondary often adopts the nickel positive electrode used in a nickel-hydrogen battery or a nickel-cadmium battery, which is a flexible foil electrode formed by generally using a substrate of a nickel foil or foamed nickel, and coating the surface or inside of the substrate with active material for positive electrode, that is, Ni(OH)2.
During the preparing and charging/discharging process of Ni(OH)2, there are always some unreduced Ni (III) ions, which are referred as electron defects in the field of semiconductor, and some stoichiometrically excessive O2− ions, which are referred as proton defects. Therefore, in the lattice of Ni(OH)2 some amount of OH− ions are replaced by O2− ions. The conductivity of this kind of semiconductor depends not only on the motility of the electron defects and the concentration of the electron defects in the lattice, but also on the above defects existed in the lattice. These reasons lead to poor conductivity of nickel hydroxide, as well as the phenomenon that oxygen is evolved soon after charging, penetrated the membrane separator, and adsorbed on the negative electrode. Therefore, Co2+ is often added to current nickel electrode to improve the conductivity of the nickel electrode, so that the reaction product Co3+ can form an excellent conductive net among the particles of nickel hydroxide with the proceeding of charge, so as to improve the “overpotential for oxygen evolution” of the nickel electrode, reduce the resistance of the battery, delay the evolution of oxygen, and enhance charging efficiency. Similarly, it facilitates improving the discharging depth of the nickel electrode during discharging, thereby improving the discharging capacity of the battery. Additionally, it is often necessary to add Cd2+ to Ni(OH)2, so as to improve the electrolytically charging overpotential of the battery.
However, if the above nickel electrode components are used in a nickel-zinc electrode, Co2+, which is easily dissolved in the electrolyte KOH solution, will diffuse toward the negative electrode, and is rapidly reduced on the zinc electrode. Due to low potential for hydrogen evolution of Co, the precipitated Co on the negative electrode will consist a hydrogen-evolving corrosive primary battery together with the electrolyte and the zinc on the negative electrode, causing the continuous evolution of hydrogen and the soaring of the pressure in the battery, and increasing the possibility of explosion and liquid leakage in the battery, leading to large safety risk. At the same time, with the continuous consumption of the negative electrode and the fading of the capacity, the charge retention property becomes poorer. Additionally, the Cd added will cause pollution to the surrounding environment and the human body, decreasing the environmental friendliness of the battery.